The Strontium Isotope Geochemistry of the Cenozoic Ocean

Abstract

A detailed record of the 87Sr/86Sr variations in seawater over the last 100 Ma has been determined by measuring the 87Sr/86Sr of well-preserved and well-dated fossil foraminifera from twelve Deep Sea Drilling Project cores of overlapping ages. Diagenetic alteration of the sample foraminifera was evaluated in three ways: by Sr/Ca ratios, by pore water chemistry, and by visual examination under the scanning electron microscope. Age assessments were made on the basis of magnetobiostratigraphy. Foraminiferal 87Sr/86Sr measurements show an almost continuous increase in the seawater 87Sr/86Sr from 0.70738 at 100 Ma to 0.70920 today. The rise has not been linear. Long-term features of the Sr isotope-age curve show that the seawater 87Sr/86Sr increased rapidly and nearly linearly between 100 and 70 Ma. After 70 Ma, the ratio increases across the Cretaceous/Tertiary boundary and then decreases to a value of about 0.70775 at 45 Ma. A second, sharper rise in the seawater 87Sr/86Sr occurs between the late Eocene and the early middle Miocene, followed by a period between 2 and 14 Ma of a more gradual increase in the 87Sr/86Sr. Superimposed on the long-term changes are smaller, short-term fluctuations including (i) a plateau of seawater 87Sr/86Sr at 12 Ma; (ii) periods of relatively rapid rise of the 87Sr/86Sr ratio beginning at 40, 22, 16 and 1 Ma (Eocene-Oligocene boundary, early Miocene, early middle Miocene, and the late Pliocene-Pleistocene, respectively); and (iii) a sharp increase in the 87Sr/86Sr of 1 x 10-4 forming a spike-like peak at the Cretaceous-Tertiary Boundary. The sharp increase in the 87Sr/86Sr at the K-T boundary implies a sudden addition of radiogenic 87Sr. An obvious source is the impact of a meteorite. Iridium anomalies, contemporaneous with mass biological extinctions at the boundary, have already suggested that the collision of a bolide was responsible for the catastrophic termination of the Mesozoic era. However, mass balance calculations show that neither the Sr content of a bolide alone nor Sr resulting from the vaporization of continental crust during the impact of a bolide on land are sufficient to explain the 87Sr/86Sr increase at the K-T boundary. Unlike previously published 87Sr/86Sr seawater curves which have suggested that the sea water isotopic composition changed monotonically during the latest Paleogene, our data show a distinct change in slope between the early Oligocene (36 to 27 Ma) and the late Oligocene to earliest Miocene (27 to ~20 Ma). The slope of the early Oligocene segment is